What is electronegativity?
Electronegativity is described as the tendency of an atom to attract electrons. So when atoms are bonded together, the atom with higher electronegativity will attract the electrons towards itself, causing that side of the molecule to be more negative.
For example, in a water molecule, the oxygen atom has a higher electronegativity(3.5) (because it has more protons to attract electrons) than the hydrogen atoms (2.1). It attracts the electrons, pulling them away from the hydrogens.
This makes the oxygen atom slightly negative (because of the extra electrons near it) and the hydrogen atoms slightly positive (since its electrons are now farther away).
polarity is when there is positive side and a negative side of the molecule. Think about the poles of a magnet there is a difference in magnetic direction. Polarity in a molecule is similar. there is a difference in electron charge, and direction.
Each atom has an electronegativity which is a measure of the strength of the pull that the atom has on electrons. If there is little or no difference between the electronegativity of the atoms in the bond the electrons are pulled equally toward both sides of the bond. However is there is a large difference in electronegativity between the two atoms the electrons are pulled toward the atom with the greater electronegativity.
The atom with the greater electronegativity will have a greater electron density around it creating a negative charge. The atom with the lower electronegativity will have less than the normal electron density around it creating a positive charge.
So differences in electronegativity create differences in electron density which creates polarity.